This invention relates generally to apparatus for controlling the blending of a solid substance with a liquid substance and more particularly, but not by way of limitation, to apparatus for remotely manually or automatically controlling, from a frac van, the delivery, via a screw conveyor, of a solid particulate additive into a mixing tank into which a pregelled liquid is concurrently flowed so that a fracturing slurry having a desired density is provided at the output of the mixing tank.
Various techniques or systems are used in the oil industry for increasing the production of oil or gas wells. One method used for stimulating a well is known as "hydraulic fracturing". During a typical hydraulic fracturing operation, a slurry of a viscous base fluid and a solid particulate material, particularly referred to as a proppant, is pumped downhole at sufficient pressures to fracture open the well's producing formation. Once a fracture has been created, the pumping of the slurry is preferably continued until a sufficient volume of the solid proppant has been carried by the slurry into the formation. After a suitable time, the pumping operation is stopped whereupon the fracture closes only so far as allowed by the proppant residue, thereby propping open the fracture in the formation. Thus, the effective flow area from the producing formation has been enlarged, thereby increasing the well's production.
During such a fracturing operation, it is important that the correct amount of the solid propping material be blended with the base fluid to form a slurry having the desired fluid density, which density results from the concentration of the proppant added to the base fluid. If the proppant concentration of the fracturing slurry is less than is needed, the height and depth of the fracture created in the producing zone will be less than desired, thereby reducing the effectiveness of the fracturing operation. If the proppant concentration of the fracturing slurry is greater than is needed, the excess proppant may plug the fracture closed at the well bore (referred to as "screen out"), thereby prematurely ending the fracturing operation and perhaps damaging, rather than enhancing, the well's production.
When such a fracturing operation is currently done with a standard blending trailer, such as of a type used by Halliburton Services, a manual control system is used by the operator of the blender to ratio the delivery of the proppant and the base fluid entering the mixing tub to try to obtain a slurry with the correct proppant concentration and thus the correct slurry density. Using this manual control system, the blender operator watches a meter indicating the flow rate of the base fluid and manually adjusts a control to vary the proppant delivery rate in a manner to match the needle position of a sand delivery rate meter with the needle position of a clean flow rate meter. This manual adjustment can be required constantly if the base fluid delivery rate is constantly fluctuating, which can easily result from a fluctuating downhole flow rate of the slurry or a fluctuating base fluid flow rate brought about by an automatic fluid level control system controlling the fluid level in the mixing tub, for example. If the fluctuations are rapid enough or if several proppant delivery devices are simultaneously controlled, an operator trying to make such manual changes in the proppant delivery rate or rates may be unable to make them fast enough to produce optimum concentrations of the proppant in the resultant slurry. Furthermore, it is difficult for an operator to control manually the proppant to obtain desired variations in concentrations over time, such as may be required for ramped or curvilinear concentrations needed to implement an optimally designed fracturing operation. Therefore, under manual control, the resultant slurry may have too low or too high a concentration of proppant which can thereby decrease the effectiveness of the fracturing operation or under extreme circumstances cause a screen out. In general, the quality of the slurry may be less than optimum in such a manually controlled fracturing operation.
In a type of fracturing control system which relies primarily on a density reading provided from a densimeter connected into the outlet flow line from the mixing tub, an optimally designed fracturing program requiring ramped or curvilinear responses is difficult to implement because the density reading yields information having a time lag relative to the quantity of proppant then being added at the time the density reading is being given. That is, control based on a signal derived from a flow out of the mixing tub has an inherent time lag because by the time the outlet flow is monitored, more blending has already occurred in the mixing tub. Therefore, a control system based on monitoring an outlet flow from the mixing tub is less responsive than a control system based on monitoring the flow into the mixing tub.
In view of these shortcomings resulting from manual control or outlet density control of a standard blender, there is the need for an improved control system which can more accurately maintain a constant proppant concentration in a fracturing slurry regardless of the fluctuations in the base fluid, thereby providing a slurry with a constant density. The system should be constructed so that multiple concentration set points can be implemented to change the proppant concentration whereby ramped or curvilinear responses can be implemented. The system should be capable of operation from a remote location in either an automatic mode or a manual mode. The system should be designed so that it can be retrofitted to a preexisting blender. To accommodate density readings in fine-tuning the primary control of the proppant delivery rate based on the inlet flow, the improved system should also be capable of use with a slurry density measuring device.